1. Field of the Invention
This invention relates to stacked beam radars, particularly to calibration of a plurality of receiver channels in the radar.
2. Description of the Prior Art
Detection of objects in the vicinity of an observer may normally be accomplished by employing a pulse-echo system, i.e. radar, electro-optical, sonar, etc. which emits pulses. The outgoing pulses are reflected by an object back to the system, the received echoes or signals, and are processed by the system to provide certain data regarding the relative position of the object, such as its range, azimuth, and elevation, with respect to the observer's system. An example of such a system is a stacked beam radar employed as a three dimensional (3D) air surveillance radar, to determine simultaneously the range, azimuth, and elevation of each object, i.e., target, detected. The "stacked beam" refers to the use by this radar of several overlapping receiving antenna beams which are displaced vertically. A description of the stacked beam radar is found in the book entitled "Radar Handbook", by Merrill I. Skolnick, Editor-in-Chief, McGraw-Hill Book Company, 1970, pages 22-4 and 22-5.
In a stacked beam radar, the reflected target signal is received simultaneously by one or more of the beams of the antenna response pattern and is processed by each of the receiving channels associated with its respective antenna beam. One of the principal elements of the computation of the height of a target is the accurate determination of the elevation angle to the target with respect to the antenna system reference axis. An amplitude monopulse measurement is used to compare the relative target signal amplitude in each of the several beams. Relating these measured relative amplitudes to the previously measured antenna beam patterns allows calculation of the elevation angle of the received target signal. The accuracy of the elevation angle in dependent on each channel producing a signal which accurately represents the target signal amplitude received in adjacent overlapping beams of the stacked beam antenna pattern.
For example, in one stacked beam radar system, the center line between two adjacent overlapping beams is considered the base angle, .theta..sub.b. From the target returns, the two adjacent overlapping beams indicating the largest target amplitude are selected as being closest to the target direction. The target is then considered to be along the base angle .theta..sub.b, or the center line between the two selected beams, plus or minus an interpolation angle, .theta..sub.i. The interpolation angle is determined by subtracting the amplitudes of the target signal in one beam from the other beam of the selected beam pair. Variations in the amplitude difference of the target signal of the selected beam pair will result in corresponding variations in the interpolation angle, .theta..sub.i. The target may very well be at a position indicated by the interpolation angle plus the base angle with respect to the antenna, or the target may be at a different angle than the interpolation angle due to differences in the gain of the receiving channels of the radar.
During operation of the stacked beam radar, it is assumed that each receiving channel has identical amplitude characteristics. However, this assumption is not usually true since it is difficult to design and build identical receiving channels which operate over wide dynamic range, 70 dB, incorporating logarithmic amplifiers which are difficult to maintain during field operations where temperature changes normally cause differential gain changes between receiving channels.
Built-in monitoring and alignment subsystems have been used to facilitate manual adjustment of amplifiers in the receiving channel. Skilled technicians have been required to make precise adjustments in one or more receiving channels to achieve alignment between channels. However, realignment is required to maintain acceptable accuracy, such as several times a day or more frequently depending upon the changes in ambient temperature. For example, current height receiving channels in a stacked beam radar are intended to attain a .+-.0.25 dB accuracy over a 70 dB dynamic range of target signals. Experience has shown, however, that .+-.0.25 dB accuracy is difficult to achieve and maintain over a wide dynamic range which would result in approximately .+-.600 feet height error at 150 nautical miles range.
In U.S. Pat. No. 3,471,855, issuing on Oct. 7, 1969 to R. T. Thompson, a system is described for generating coherent calibration test signals for an array of receiver channels to measure the gain and phase stability of each of the channels over their operating frequency range. In U.S. Pat. No. 3,471,855 each receiver operates at a different frequency. Calibration signals are derived from a common reference source in a plurality of phase locked synthesizers to provide a plurality of coherent signals. The calibration signals are coupled to the inputs of the receivers with their respective outputs coupled to phase and amplitude comparators which compares the outputs to the initial calibrating waveform. The magnitude of the amplitude and phase error was measured by using a phase shifter and a variable precision digital attenuator to modify the reference calibration signal to the comparator until the amplitudes and phase matched with the signal from the receiver system. The adjustment setting on the phase detector and the digital attenuator provided a measure of the phase and amplitude error.
In U.S. Pat. No. 3,361,972, which issued on Jan. 2, 1968, to J. L. Eaves, a system for automatically adjusting the log slope of the input versus output curve of log receivers in a multiple channel search type radar is described. Automatic slope adjustment in each channel is achieved by injecting two test pulses into the log receiver of each channel every 3,000 microseconds. The log slope of the log receiver initially is set to a preselected value and if for some reason the slope of the log receiver deviates from its preselected value, an error signal will be generated to increase or decrease the log slope of the log receiver to return it to the preselected value. By using test pulses, each receiving channel is adjusted to a preselected value in an attempt to attain absolute matching of the characteristics of two or more receivers. A motor control circuit is used to adjust an attenuator to bring each receiver log slope into alignment with the next.
A self-calibration system utilizing a memory is described in U.S. Pat. No. 4,017,856 which issued on Apr. 12, 1977 to R. J. Wiegand and is assigned to the assignee herein. In the patent to Wiegand a microwave transponder receives RF signals which are converted into a voltage which drives a voltage controlled oscillator to generate an output RF signal of the same frequency as received. The microwave transponder is calibrated to compensate for temperature, oscillator drift and component variations by utilizing an instantaneous frequency discriminator which generates two video signals used for generating a memory address signal which is used to address a memory. The memory output voltage tunes a microwave voltage controlled oscillator to provide an RF output signal. A counter and a digital to analog converter provides voltages to the voltage controlled oscillator which tunes the voltage controlled oscillator to a corresponding microwave output signal. The output signal is coupled to the instantaneous frequency discriminator which generates a memory adjust signal whereupon the digital value from the counter is stored in a location corresponding to the address derived from the instantaneous frequency discriminator. The counter provides a sequence of increasing or decreasing test pulses for generating a wide range of voltages which in turn are stored in the memory at the memory address derived from the instantaneous frequency discriminator.
It is therefore desirable to provide a method of self-calibration for electronic systems which have inherent difficulty in obtaining the required degree of accuracy through design of their performance characteristics, such as the amplitude response of a plurality of receiving channels operating over a wide dynamic range of received target signals.
Furthermore, it is desirable to provide self-calibration of a plurality of receivers which have differences in gain over the desired dynamic range of target signals by utilizing a look-up table.
Furthermore, it is desirable to provide self-calibration by generating test pulses of predetermined amplitude which may be injected into each receiver channel.
Furthermore, it is desirable to make accurate relative amplitude measurements utilizing a common test pulse injected into a plurality of receivers with the amplitude of the output signals stored in a table along with the amplitude of the test pulse.